In a four-wheel drive motor vehicle, since the trajectories of the front wheels and the rear wheels deviate from each other during a cornering action of the vehicle, some difference arises between the rotational speeds of the front wheel set and the rear wheel set. Therefore, in a four-wheel drive motor vehicle in which the front wheel set and the rear wheel set are directly connected to each other or connected by way of a viscous coupling, it is known that the so-called tight turn braking may occur and some difficulty may be encountered in driving the motor vehicle when a tight turning action is made in a very low speed range. To the end of eliminating this problem, Japanese Patent Laid Open Publication No. 59-216766 proposed to switch the state of the motor vehicle from a four-wheel drive condition to a two-wheel drive condition when the steering angle has exceeded a certain prescribed value. However, since the advantages of four-wheel drive is lost during the cornering action, this technology does not necessarily offer a satisfactory solution to this problem.
U.S. Pat. No. 4,681,180 proposes the use of a pair of hydraulic clutches of a variable transmission torque type for improving the dynamic lateral response of a rear wheel drive vehicle during a cornering action by supplying more torque to the outer rear wheel than to the inner rear wheel in a low speed range and by supplying more torque to the inner rear wheel than to the outer rear wheel in a high speed range. A steering sensor is used to detect the direction of the cornering action.